Upgrade our copies of clang, llvm, lld, lldb, compiler-rt and libc++ to
3.9.1 release.
Please note that from 3.5.0 onwards, clang, llvm and lldb require C++11
support to build; see UPDATING for more information.
Release notes for llvm, clang and lld will be available here:
<http://releases.llvm.org/3.9.1/docs/ReleaseNotes.html>
<http://releases.llvm.org/3.9.1/tools/clang/docs/ReleaseNotes.html>
<http://releases.llvm.org/3.9.1/tools/lld/docs/ReleaseNotes.html>
Relnotes: yes
MFC after: 2 weeks
X-MFC-with: r309124